Hypoglycaemia is the most common complication experienced by patients with insulin dependent diabetes mellitus. Its onset is characterised by symptoms which include sweating, tremor, palpitations, loss of concentration and tiredness. Although the majority of patients can use these symptoms to recognise the onset of hypoglycaemia and take corrective action, a significant number of patients develop hypoglycaemic unawareness and are unable to recognise the onset of symptoms.
Concerning hypoglycaemia, the blood glucose in men can drop to 3 mmol/L after 24 hrs of fasting and to 2.7 mmol/L after 72 hrs of fasting. In women, glucose can be low as 2 mmol/L after 24 hrs of fasting. Blood glucose levels below 2.5 mmol/L are almost always associated with serious abnormality. Hypoglycaemia in diabetic patients has the potential to become dangerous. In many cases of hypoglycaemia, the symptoms can occur without the awareness of the patient and at any time, eg. while driving or even during deep sleep. In severe cases of hypoglycaemia, the patient can lapse into a coma and die. Nocturnal episodes are also potentially dangerous and have been implicated when diabetic patients have been found unexpectedly dead in bed. Hypoglycaemia is one of the complications of diabetes most feared by patients, on a par with blindness and renal failure.
Current technologies used for diabetes diagnostic testing and self-monitoring are known. For example, glucose meter manufacturers have modified their instruments to use as little as 2 μl of blood and produce results in under a minute. However, devices which require a blood sample are unsatisfactory in that the sample is painful to obtain, and continuous monitoring is not possible.
There are only a few manufacturers who have produced non-invasive blood glucose monitoring systems. The Diasensor 1000 from Biocontrol Technology Inc. uses near-infrared technology and multivariate regression to estimate blood glucose levels. The system is very expensive, it has to be individually calibrated to each patient, it has to be recalibrated every three months, and the calibration process takes up to seven days. The GlucoWatch monitor from Cygnus is designed to measure glucose levels up to three times per hour for 12 hours. The AutoSensor (the disposable component) which is attached to the back of the GlucoWatch monitor and adheres to the skin will provide 12 hours of measurement. The product uses reverse iontophoresis to extract and measure glucose levels non-invasively using interstitial fluid. It requires 12 hours to calibrate, only provides 12 hours of measurement, requires costly disposable components, and the measurement has a time delay of 15 minutes. Another device, the Sleep Sentry, monitors perspiration and a drop in body temperature to alert the patient to the onset of hypoglycemia. In studies of patients admitted for overnight monitoring it was found to be unreliable in between 5% and 20% of cases.
During hypoglycaemia, the most profound physiological changes are caused by activation of the sympathetic nervous system. Among the strongest responses are sweating and increased cardiac output. Sweating is centrally mediated through sympathetic cholinergic fibres, while the change in cardiac output is due to an increase in heart rate and increase in stroke volume.
It is an object of the invention to provide a non-invasive method of detecting medical conditions in patients, which is relatively accurate and relatively inexpensive to use, and will trigger an alarm signal within an acceptable time delay from when a condition which is being monitored for presents itself